Many computer programs, activelly used in the world are based on the open source philosophy. When some natural need occurs, users have the possibility to improve those open source products and/or constantly change some of their elements.
Some successful open source examples: OpenOffice, Inkscape, Mozilla Firefox, Evolution, Tomboy.
Firefox
Mozilla Firefox is a cross-platform browser, providing support for various versions of Microsoft Windows, Mac OS X, and Linux. However, the source code has been unofficially ported to other operating systems, including FreeBSD,OS/2, Solaris, RISC OS, SkyOS, BeOS and more recently, Windows XP Professional x64 Edition.
Firefox's source code is available under the terms of the Mozilla tri-license (MPL/GPL/LGPL) as free and open source software.
http://www.mozilla.org/firefox
Inkscape
Inkscape is an Open Source vector graphics editor, with capabilities similar to Illustrator, Freehand, CorelDraw, or Xara X using the W3C standard Scalable Vector Graphics (SVG) file format. Supported SVG features include shapes, paths, text, markers, clones, alpha blending, transforms, gradients, patterns, and grouping. Inkscape also supports Creative Commons meta-data, node editing, layers, complex path operations, bitmap tracing, text-on-path, flowed text, direct XML editing, and more. It imports formats such as JPEG, PNG, TIFF, and others and exports PNG as well as multiple vector-based formats.
Inkscape's main goal is to create a powerful and convenient drawing tool fully compliant with XML, SVG, and CSS standards by using open, community-oriented development.
http://www.inkscape.org/
OpenOffice
OpenOffice.org the product is a multi-platform office productivity suite. It includes the key desktop applications, such as a word processor, spreadsheet, presentation manager, and drawing program, with a user interface and feature set similar to other office suites. Sophisticated and flexible, OpenOffice.org also works transparently with a variety of file formats, including those of Microsoft Office, and the vendor-neutral OpenDocument standard from OASIS.
Available in over 65 supported languages with more being constantly added by the community, OpenOffice.org runs stably and natively on Solaris, Linux (including PPC Linux), Windows, Mac OS X (X11), and numerous other platforms.
Evolution
Evolution is the official personal information manager and workgroup information management tool for GNOME. It combines e-mail, calendar, address book, and task list management functions. It has been an official part of GNOME since version 2.8 in September 2004. Evolution development is sponsored primarily by Novell.
Its user interface and functionality are similar to Microsoft Outlook. It has some distinguishing features: iCalendar support, full-text indexing of all incoming mail, powerful email filters writable in Scheme, and a "Search Folders" feature (i.e., saved searches that look like normal mail folders).
www.gnome.org/projects/evolution/
Tomboy
Tomboy is an open-source desktop notetaking application for Linux and Unix. It uses a Wiki-like linking system to connect notes together. Tomboy is part of the GNOME desktop environment and is licensed under the GNU Lesser General Public License. Tomboy is often used for personal information management. The main principle is a notepad. There is a main page in which the user puts all links to other pages. Words to pages that are notes automatically become hyperlinks, allowing for the management of large libraries of personal information, such as references to favorite artists that would then automatically be highlighted in e-mails containing their names.
http://www.wikipedia.org